Timeline for appearances of persons or entities

[vthorsteinsson]

Add the ability to display a graphical timeline of appearances (in articles) of persons or entities. The timeline should be zoomable from a level of years down to days. For example, the user should be able to select Bjarni Benediktsson and see a graphical timeline of his frequency of appearances in news articles, zoomable down to individual articles (by headline). It might be interesting to allow two or more such timelines to be displayed in parallel (at synchronized zoom levels) for comparison purposes.